Always i have to activate license win 7 gss 2.5

Hello.

I have several hp z400 workstation with Windows vista. We can upgrade from vista to win7 when appear this new operating system thanks to campaign of microsoft to upgrade.

Now we install win 7 and activate with a serial, then clone to usb hardisk with gss 2.5 client and when we restore image to another machines z400 we always have to activate win 7 by phone.

Any idea?

Thanks.